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Saturday, June 18, 2016

A Very Different Late Spring (and a Wet Saturday)

Right now (Saturday AM), showers are found over much of the Northwest, with particularly heavy rain along the eastern slopes of the Cascades (see radar)

The latest WSDOT cam shot at Ryegrass Summit near Vantage, says it all.

The current infrared satellite image shows the circulation of a low center west of the northern Oregon coast and an impressive band of clouds over Washington and Oregon. The frontal band should sweep northward during the day, but showers will follow in its wake. Sunday should be much drier and warmer--allowing fathers to enjoy some deserved outdoor activity.

But what is striking to me is how different late spring 2016 is from the same time last year, with important implications for wildfire danger and drought issues. The plots below show temperatures at Seattle Tacoma Airport for the last four weeks, this year and last (since roughly May 20th).

This year temperatures were very normal, except for the hot spell in late May and early June. A number of days got to the normal minimum temps.

But last year was very different. Hot periods dominated and most days did not get even near the normal minima. The warm BLOB of high sea surface temperatures over the eastern Pacific undoubtedly contributed to the later.

Rainfall? This year has been dry, but we have gotten about 60% of normal precipitation for this 4-week period. Last year, maybe 15% of normal.

Why so different? Last year there was persistent upper level ridging (high pressure) over the West Coast, while this year the high pressure has moved over the central Pacific (and the interior of North America), leaving a trough (low pressure) over the northern and central West Coast. Let me show you the forecast upper level maps (500 hPa) for today (11 AM), Monday, and Friday.

You notice the similarity? Troughing over our region.

The ending of El Nino has led to a substantial reorganization of the atmospheric circulation, and it appears that this change will persist at least through the end of this week. The fact that eastern WA is getting a good wetting in mid June will push any potential fire season into the future, which is good. Cooler weather has greatly reduced water demand.

So the rain is good news, except for one group: the nude bicyclists that are expected at Seattle's Solstice parade today. Waterproof body paint is recommended.

7 comments:

This is the June I remember so well over the decades. It is sort of nice to see its return for a year. It is certainly a welcome break in not having to water my vegetable garden twice a day. Great for the cool weather crops such as lettuce...

Forgive me if this has been covered previously, but this is where I get confused. If Washington has only received %60 of total precip this year, then why do sites like SNOTEL show YTD over 100%? Is it because of the different beginning and ending dates used, like the "water year" officially ending on June 30th?

Not 60% of normal for the calendar year, 60% of normal for spring, which began April 1 or so. We're still above normal for both the calendar year and the water year. But the spring specifically has been a bit drier than normal around Seattle. Notice all the dry yellow grass. We got almost an inch of rain yesterday in Shoreline though so we're looking good up here!